In recent years, a large amount of data needs to be collected and processed. Thus, a large amount of information is processed within a limited space. Therefore, the heating density of electronic components such as a CPU (central processing unit) increases. Electronic components cannot effectively demonstrate their effects (performance) unless they are cooled. In some cases, if an electronic component is not cooled, it is broken to cause many problems. A cooling apparatus that utilizes a phase change of a refrigerant to transport and diffuse heat for cooling (hereinafter referred to as “phase change cooling apparatus”) has been proposed as means for cooling an electronic component having a high heating density.
Next, an operation of the phase change cooling apparatus will briefly be described. The phase change cooling apparatus comprises a heat receiving section that receives heat from a heating element of an electronic component such as a CPU, a heat radiation section that radiates the transported heat by using a phase change of a refrigerant, and a pipe connecting the heat receiving section and the heat radiation section to each other.
The heat receiving section is thermally connected to the heating element with heat conduction grease. The heat radiation section is provided with an externally provided cooler such as a cooling fan. Thus, heat radiation is promoted from the heat radiation section to the air.
In the heat receiving section that receives heat from the heating element, a liquid phase refrigerant in the heat receiving section boils with the heat transmitted from the heating element, so that the liquid phase refrigerant changes in phase into a gaseous phase refrigerant. When the liquid phase refrigerant changes in phase into a gaseous phase refrigerant, the refrigerant absorbs therein the heat as latent heat. Because a gaseous phase refrigerant has a lower density than a liquid phase refrigerant, the gaseous phase refrigerant ascends due to its buoyancy and moves to the heat radiation section through a gaseous phase pipe. Since the buoyancy is used to move the gaseous phase refrigerant to the heat radiation section, the heat radiation section needs to be located at a vertically upward position with respect to the heat receiving section.
The gaseous phase refrigerant that has moved to the heat radiation section radiates heat to the air with cooling air delivered from a cooling fan externally provided. Thus, the gaseous phase refrigerant changes in phase into a liquid phase refrigerant. Since a liquid phase refrigerant has a higher density than a gaseous phase refrigerant, the liquid phase refrigerant descends due to the gravity and returns to the heat receiving section through a liquid phase pipe. The returned liquid phase refrigerant is supplied with heat and reused for circulation of the refrigerant.
In this manner, the phase change cooling apparatus uses the phase change of a refrigerant and can thus circulate the refrigerant without any pump. Furthermore, the amount of heat per unit mass that can be transported by the phase change is as large as several hundred times the amount of heat per unit mass that is obtained by a method of transporting heat with a temperature increase of a refrigerant such as water-cooling. Therefore, the method using the phase change is suitable for transporting and cooling heat having higher calorific values.
Although the phase change cooling apparatus is suitable for transporting and cooling heat having higher calorific values, recent high densification of electronic devices has required higher performance of the phase change cooling apparatus. In order to use a phase change cooling apparatus for highly densified electronic devices, the entire phase change cooling apparatus should not be increased in size because electronic devices have only limited areas for a cooling apparatus. Accordingly, it is desired to enhance the performance of the heat receiving section more than the heat radiation section, which greatly depends upon a radiation area. In order to enhance the performance of the heat receiving section, it is important to boil a large amount of a liquid phase refrigerant without an increased difference in temperature between the boiling section, which receives heat from the heating element and boils a liquid phase refrigerant into a gaseous phase refrigerant, and the gaseous phase refrigerant.
Various types of boiling means with a small temperature difference have heretofore been proposed.
For example, in FIG. 12(b) of Patent Literature 1, a spherical boiling section structure with an opening portion is used. This structure has a wedge-shaped portion, which is effective for boiling, to form irregularities that are effective for formation of bubble cores. Thus, the structure increases the number of bubbles generated for improvement. Generally, in order to form bubble cores, bubbles are trapped by this wedge-shaped portion, so that the bubbles press a liquid film against an inner wall of the wedge-shaped portion to make the liquid film thinner. The thinned liquid film quickly boils with a small temperature difference. Therefore, the wedge-shaped portion by which the bubbles are trapped serves as a bubble core generation portion.
FIG. 5 of Patent Literature 2 shows that a surface of a fin-like groove structure is covered with a thin porous member by electroplating or the like so as to increase the friction of the surfaces of the grooves. Thus, bubbles are trapped so that a liquid phase refrigerant is pressed against wall surfaces of the grooves by those bubbles to form a thin liquid film. When heat of a heating element is applied to the thin liquid film on the surfaces of the grooves, the liquid phase refrigerant can boil quickly, which improves the cooling capability.